Tuareg rebels clashed with Islamist fighters overnight in northern Mali, a vast swathe of land that has been held by disparate rebel forces for the last two months, witnesses said Friday. The fighting between the tribal National Movement for the Liberation of Azawad (MNLA) and the Ansar Dine Islamists came after the two erstwhile allies fell out over whether Sharia, or Islamic law, should be applied in the region, observers said. The clash near Kidal, a remote regional capital in the northeast, involved automatic weapons, an official told AFP by telephone. "There was a lot of gunfire," he said, adding: "I saw a lot of cars cross the city." Calm had returned by dawn, the official said, but he noted that several MNLA flags had been removed from around the city. A city resident confirmed that there had been heavy gunfire. "Ansar Dine were to the north of Kidal, a group of MNLA to the south," he said. Tuareg rebels fought alongside Ansar Dine in January to conquer the entire northern half of Mali virtually unopposed in March, taking advantage of chaos in the capital Bamako when renegade soldiers toppled then-president Amadou Toumani Toure in a coup.